Initial commit.

- Minimal documentation system.
- Minimal bundler setup.
- Minimal rake setup.
- License.
- Beginning of documentation.
Add first implementation and test.
Use #safe_constantize where appropriate.
Add merge, merge!, dup and has_property? to ObjectSchema.
Allow to "re-open" object schema definition.
More robust semantics for in-place validation.
Only Grant read access to the last error.
Make sanitize_doc public and update documentation.
Simplify CompositeSchema implementation.

composed_by and sanitize class method where pure sugar that we can
get rid of because they really add no value.
Sanitized object doc always have indifferent access.

This simplify CompositeSchema#validate implementation.
Rewrite schema documentation system.

We no longer have documentation system based on the heavy metadata system.
Instead, we have a single option "doc". Its text contains both the title
and the description like in Git's commit message.
Implement "nodoc" feature.
Add Schema#documented?.
Add ObjectSchema#documented_properties.

And test it.
Write "Take a tour!" section.
Add "Getting started!" section to README.
Fix English.
Use to_s for DslDumper and to_json for JsonSchemaDumper.

Also add inspect.
Add Schema#to_pretty_json and test it.
Convert "releated work" section to Markdown.